Wireless communication devices have become smaller and more powerful as well as more capable. Increasingly users rely on wireless communication devices for mobile phone use as well as email and Internet access. At the same time, devices have become smaller in size. Devices such as cellular telephones, personal digital assistants (PDAs), laptop computers, and other similar devices provide reliable service with expanded coverage areas. Such devices may be referred to as mobile stations, stations, access terminals, user terminals, subscriber units, user equipments, and similar terms.
A wireless communication system may support communication for multiple wireless communication devices at the same time. In use, a wireless communication device may communicate with one or more base stations by transmissions on the uplink and downlink. Base stations may be referred to as access points, Node Bs, or other similar terms. The uplink or reverse link refers to the communication link from the wireless communication device to the base station, while the downlink or forward link refers to the communication from the base station to the wireless communication devices.
Wireless communication systems may be multiple access systems capable of supporting communication with multiple users by sharing the available system resources, such as bandwidth and transmit power. Examples of such multiple access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, wideband code division multiple access (WCDMA) systems, global system for mobile (GSM) communication systems, enhanced data rates for GSM evolution (EDGE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Mobile devices require an accurate current to correctly operate the device. In particular, an accurate current is needed for modem operation, which provides for calling and other advanced features. As chips become more complex, with more features combined within one die, such as the system on chip (SoC), both layout and current control become more challenging. An additional challenge is that more features must be routed using a limited number of pins, which makes device testing more difficult as many tests require the same output pins.
Mobile devices are also becoming increasingly popular, with many relying on them in place of conventional landlines. With increase use and popularity, it is important to test and produce mobile devices in the most expeditious and cost-effective manner. One area that currently limits automation is calibrating the reference current of the transmit digital to analog converter (DAC). At present, the reference current is calibrated using cumbersome analog and digital techniques to calibrate the current. These current techniques are designed to ensure a full-scale output current at a specified value, typically 2 mA. The transmit DAC output current full-scale value must be accurate within a specified value, and often requires very high precision of +/−1%. This accuracy may vary depending on the device and the operating system. A SoC may require this level of accuracy because the reference current generation may be done on the mobile station modem (MSM) side, (inside the transmit DAC) function, without any knowledge of information from the SoC system. Because the MSM may include process variations arising from variations from the incorporated resistors it is necessary to calibrate the full-scale output current on the transmit DAC. The variation in those resistors may be quite significant and require considerable individual adaptation in order to generate a reference current of the desired level of accuracy.
Previously, this variation was addressed by selectively blowing fuses to correct the on-chip resistor to match a known, external golden resistor. Once the resistor is tuned, it is then used as part of the band gap current generation circuit that generates a band gap reference current. A current mirror circuit would then be used with different multiplication ratios to multiply the input band gap current to the desired final reference current for the transmit DAC.
An alternate method also used previously involved blowing fuses to match a known, external golden resistor. Once the resistor is tuned, it is then used as part of a voltage circuit (V2I) to generate an accurate reference current. A current mirror circuit is then used with different multiplication ratios to multiply the reference current to the final desired reference current for the transmit DAC.
Each of the above methods has disadvantages. The methods require both hands-on digital and analog techniques that require individual adjustment on each chip. This increases time and cost. The methods may also require additional pins for testing and require routing traces to resistors that must be located nearby for greatest accuracy, which adversely affects the circuit board size and may also result in temperature hot spots on the circuit board. These methods also provide only limited gain adjustment and lack precision.
There is a need in the art for a method of digitally calibrating the reference current input of the transmit DAC to provide an accurate current within specified bounds.